1. Field of the Invention
The present invention relates to an image forming system comprised of an image forming apparatus that forms an image on a sheet, and a clear coating apparatus connected to the image forming apparatus on a downstream side of the same so as to perform clear coating on the sheet using clear toner which becomes clear (transparent) after being fixed on the sheet.
2. Description of the Related Art
In recent years, there has been an increasing demand for full-color electrophotographic image forming apparatuses ensuring high image quality. Further, it has been increasingly desired to realize a high-quality photographic tone image mode in an electrophotographic image forming apparatus, with proliferation of digital cameras as a major reason.
On the other hand, there has been proposed a clear coating apparatus that outputs a high-quality photographic tone image on a sheet and then coats the sheet with a toner image which will become clear after being fixed (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H03-13079). The clear coating apparatus can provide an image equivalent or close in glossiness to an image obtained by silver-salt photographic processing, by performing clear coating on a sheet.
There has also been proposed a technique for realizing an image forming process using Y (yellow), M (magenta), C (cyan), and Bk (black) toners and a clear coating process using a clear toner within a single image forming apparatus (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. H09-200551).
However, the conventional clear coating apparatus is not configured to be able to automatically form clear toner images on the respective opposite sides of a sheet, and hence in order to form clear toner images on the respective opposite sides of a sheet, it is required to form an image on the front side of the sheet and then manually set the sheet discharged from the apparatus on a sheet feeder again. On the other hand, the image forming apparatus incorporating a clear coating device can perform clear coating only in the apparatus itself, and therefore the clear coating device lacks versatility.
Clear toner has a property of increasing its glossiness as the number of times of heat reception increases with increase in the number of times of passage through a fixing device. This property of clear toner can cause a difference in glossiness between the front and reverse sides of a sheet when clear coating is performed on the opposite sides of the sheet. For example, in a case where a front-side image forming process, a front-side clear coating process, a reverse-side image forming process, and a reverse-side clear coating process are executed in the mentioned order, clear toner on the front side passes through the fixing device three times, whereas clear toner on the reverse side passes through the fixing device only once. Therefore, a difference in glossiness is caused between the front side and the reverse side, which causes degradation of the quality of an output.